Perhaps they overwintered in the mulch and are emerging now. My *guess* is they will move on soon. They generally feed on box elder trees, but have been observed sucking the leaves of other trees when their prefered source isn't plentiful.

I haven't heard of them bothering tomatos. Are you seeing any leaf damage? If not consider monitoring the plants and doing nothing.

If you need to eradicate them insecticidal soap is extremely effective.

Maybe just go rake the mulch around to see if that persuades them to move on otherwise my guess is they are getting ready to move out all on their own soon.